Screen

Abstract

The invention relates to a strainer for treating a fibrous material suspension suitable for producing a paper, cardboard, tissue, or other fibrous material web, having a plurality of strainer perforations, the smallest cross-sectional areas of which are approximately of the same size. The sorting effect and the throughput are to be improved such that the cross-sectional areas of the strainer perforations on the inflow side have varying size.

What is claimed is: 
     
       1. A screen for treating a fibrous stock suspension for producing a fibrous material web, the screen comprising a plurality of perforations, each of said plurality of perforations having a cross sectional area which has a smallest section with a size which is substantially the same, and at least some of said perforations including an inflow side having a cross-sectional area with a different size from one said perforation to another. 
     
     
       2. The screen according to  claim 1 , wherein a portion of said plurality of perforations enlarges toward said inflow side of the screen. 
     
     
       3. The screen according to  claim 2 , wherein said portion is approximately half of said plurality of perforations. 
     
     
       4. The screen according to  claim 2 , wherein said portion of said plurality of perforations enlarges conically toward said inflow side of the screen. 
     
     
       5. The screen according to  claim 1 , wherein said inflow side of a portion of said plurality of perforations is cylindrical in shape. 
     
     
       6. The screen according to  claim 5 , wherein said inflow side of half of said plurality of perforations is cylindrical in shape. 
     
     
       7. The screen according to  claim 1 , wherein said plurality of perforations have a circular cross section. 
     
     
       8. The screen according to  claim 1 , wherein said cross sectional area of said smallest section of each of said plurality of perforations is between approximately 0.5 and 700 millimeters square (mm 2 ). 
     
     
       9. The screen according to  claim 8 , wherein said cross sectional area of said smallest section of each of said plurality of perforations is between approximately 3 and 300 mm 2 . 
     
     
       10. The screen according to  claim 1 , wherein said plurality of perforations are arranged uniformly. 
     
     
       11. The screen according to  claim 10 , wherein said plurality of perforations are arranged in a plurality of parallel rows located adjacent one another. 
     
     
       12. The screen according to  claim 11 , wherein said plurality of perforations have a shape which alternates between a cylindrical shape and a conical shape in one of said plurality of parallel rows. 
     
     
       13. The screen according to  claim 11 , wherein said plurality of perforations are arranged in a plurality of rows located adjacent to each other and extending vertical to each other. 
     
     
       14. The screen according to  claim 4 , wherein said portion of said plurality of perforations which enlarge conically toward said inflow side enlarge by 100 to 400%. 
     
     
       15. The screen according to  claim 14 , wherein said part of said plurality of perforations which enlarge conically towards said inflow side enlarge by 200 to 300%. 
     
     
       16. The screen according to  claim 1 , wherein the screen has a thickness between 6 and 30 mm. 
     
     
       17. The screen according to  claim 4 , wherein a depth of a cone part of said plurality of perforations conically enlarging toward said inflow side is between approximately 0.5 and 5 mm. 
     
     
       18. The screen according to  claim 1 , wherein the fibrous material web is one of a paper, cardboard and tissue web.